The purpose of this multicenter, industry-sponsored study is to evaluate whether SB217242 is an effective, safe, and well tolerated drug compared to placebo in NYHA Class II and III congestive heart failure patients on standard care therapy. SB217242 is a non-selective endothelin receptor antagonist that has been shown to be active in several animal models of cardiovascular disease. Congestive heart failure is among these diseases; thus it is hypothesized that SB217242 would be effective as a long-term treatment for systolic congestive heart failure. Patients will be randomized into one of six test groups in a 3:1:1:1:1:1 ratio. Group 1 will receive placebo throughout the nine-month study protocol. Group 2 will be titrated from 15 to 30 mg of study drug over the course of the 12 week titration phase receiving the full 30 mg by week 4. Groups 3 and 4 will be titrated from 20 mg to 60 mg of study drug over the course of the 12 week titration phase. Group 3 will be titrated to 40 mg by week 4 and 60 mg by week 8. Group 4 will be titrated to 40 mg by week 2 and 60 mg by week 4. Group 5 will receive a target dose of 90 mg of study drug, titrated from 30 mg initiation dose to 60 mg at week 4, and 90 mg by week 8. Finally, group 6 will be receive a 10 mg dose of enalapril to be added to their baseline ACE inhibitor throughout the course of the nine month study protocol, and will not receive study drug.